The club revealed in a statement that the French forward will undergo further tests this week to see the full extent of the damage.
But initial reports suggest that he could miss at least the next three weeks.
Real Madrid returned to training yesterday following the La Liga winter break and all of their top players were in attendance.
But Benzema was not involved sparking speculation that he was either injured or to be sold.
However, the club later explained his absence in a statement released on their website.
The statement read: “Following tests carried out on Karim Benzema by the Sanitas Real Madrid Medical Services, he has been diagnosed with a muscular injury in biceps femoris of his right leg.
“His recovery will continue to be assessed.”
Benzema limped off in the 66th minute of their 3-0 El Clasico defeat to Barcelona on December 23.
And he could miss the next five matches against Numancia twice, Celta Vigo, Villarreal and Deportivo La Coruna.
Real are currently struggling in their domestic campaign as they sit fourth in the table, 14 points behind leaders Barcelona.
And Benzema has only scored five goals this term sparking rumours that president Florentino Perez wants to replace him in January.